Beam time was requested to perform diffraction experiments on crystals of two glutamine amidotransferase enzymes. GMP synthetase and glutamine PRPP amidotransferase. Structures have been solved for both enzymes, and high-resolution studies will be carried out to address specific questions of biological function. The enzymes have a complex organization, with two catalytic active sites that must work in a highly coordinated way to catalyze a single biochemical transformation. Diffraction experiments have been outlined with various ligand states and mutants to probe the coupling of cataltyic activities in the two active sites of each enzyme. The aim is to measure data to the highest resolution possible. A second major aim is to measure data for the high-resolution structure of the active conformation of glutamine PRPP amidotransferase, the allosteric enzyme controlling purine biosynthesis. A structure at moderate resolution has been obtained very recently for this allosteric state. However, crystals have a unit cell edge in excess of 300 , and the diffraction limit of these crystals cannot be reached with conventional radiation. Biological significance of the projects derives from the general features of coupled catalysis by complex enzymes, and allosteric structural transitions in regulatory enzymes. All crystals can be frozen.